Twin

Twins are two offspring resulting from the same pregnancy, either of the same or opposite sex.

The general term for more than one offspring from the same pregnancy is multiples, for example triplets refers to cases of three offspring from the same pregnancy. A fetus alone in the womb is called a singleton.

Human twins are two individuals who have shared the uterus during a single pregnancy and are usually, but not necessarily, born in close succession. Due to the limited size of the mother's womb, multiple pregnancies are much less likely to carry to full term than singleton births, with twin pregnancies lasting only 37 weeks on average, 3 weeks less than full term.^[1] Since premature births can have health consequences for the babies, twin births are often handled with special precautions.

There are estimated to be approximately 125 million human twins and triplets in the world (roughly 1.9% of the world population), and just 10 million monozygotic twins (roughly 0.2% of the world population and 8% of all twins).^[2] The current rate in the United States is 31 twin births per 1,000 women.^[3]

Twins can either be monozygotic (MZ, colloquially, "identical") or dizygotic (DZ, colloquially, "fraternal")

Types of twins

There are five common variations of twinning. The three most common variations are all dizygotic: (1) male-female twins are the most common result, at about 40 percent of all twins born; (2) female DZ twins (sometimes called sororal twins); (3) male DZ twins. The other variations are monozygotic twins: (4) female MZ twins and (5) (least common) male MZ twins. Male singletons are slightly, about five percent, more common than female singletons. The rates for singletons vary slightly by country as shown in the CIA World Fact book ^[4]. For example, the sex ratio at birth in the US is 1.05 male(s)/female where it is 1.07 male(s)/female in Italy. However, males are also more susceptible than females to death in utero, and since the death rate in utero is higher for twins, it leads to female twins being more common than male twins.

Another variety of twins, "polar body twins," (one egg fertilized by two different sperm) is a phenomenon that was hypothesized to occur and may recently have been proven, very rarely, to exist. Polar body twinning would result in "half-identical" twins.^[5]

Dizygotic twins

Dizygotic twins (commonly known as "fraternal" or "non-identical twins") usually occur when two fertilized eggs are implanted in the uterine wall at the same time. The two eggs form two zygotes, and these twins are therefore also known as dizygotic as well as "biovular" twins. When two eggs are independently fertilized by two different sperm cells, DZ twins result.

Dizygotic twins, like any other siblings, have an extremely small chance of having the exact same chromosome profile. Like any other siblings, DZ twins may look very similar, particularly given that they are the same age. However, DZ twins may also look very different from each other. They may be different sexes or the same sex. The same holds true for brothers and sisters from the same parents, meaning that DZ twins are simply brothers and/or sisters who happen to have the same age.

Studies show that there is a genetic basis for DZ twinning. However, it is only the female partner that has any influence on the chances of having DZ twins as the male cannot make her release more than one ovum. Dizygotic twinning ranges from six per thousand births in Japan (similar to the rate of monozygotic twins) to 14 and more per thousand in some African states.^[6]

DZ twins are also more common for older mothers, with twinning rates doubling in mothers over the age of 35.^[7] With the advent of technologies and techniques to assist women in getting pregnant, the rate of fraternals has increased markedly. For example, in New York City's Upper East Side there were 3,707 twin births in 1995; there were 4,153 in 2003; and there were 4,655 in 2004. Triplet births have also risen, from 60 in 1995 to 299 in 2004.

Monozygotic twins

Identical twins.

Monozygotic twins, frequently though incorrectly referred to as identical twins occur when a single egg is fertilized to form one zygote (monozygotic) which then divides into two separate embryos. Their traits and physical appearances are not exactly the same; although they have identical DNA, environmental conditions both inside the womb and throughout their lives influence the switching on and off of various genes. Division of the zygote into two embryos is not considered to be a hereditary trait, but rather an anomaly that occurs in birthing at a rate of about three in every 1000 deliveries worldwide,^[8] regardless of ethnic background. The two embryos develop into fetuses sharing the same womb. When one egg is fertilized by one sperm cell, and then divides and separates, two identical cells will result. If the zygote splits very early (in the first two days after fertilization) they may develop separate placentas (chorion) and separate sacs (amnion). These are called dichorionic, diamniotic (or "di/di") twins, which occurs 20-30 percent of the time. Most of the time in MZ twins the zygote will split after two days, resulting in a shared placenta, but two separate sacs. These are called monochorionic, diamniotic ("mono/di") twins.

In about one percent of MZ twinning the splitting occurs late enough to result in both a shared placenta and a shared sac called; monochorionic, monoamniotic ("mono/mono") twins. Finally, the zygote may split extremely late, resulting in conjoined twins. Mortality is highest for conjoined twins due to the many complications resulting from shared organs. Mono/mono twins have an overall in-utero mortality of about 60 percent, principally due to cord entanglement prior to 32 weeks gestation. Many times, monoamniotic twins are delivered at 32 weeks electively for the safety of the babies. In higher order multiples, there can sometimes be a combination of DZ and MZ twins.

Mono/di twins have about a 25 percent mortality due to twin-to-twin transfusion syndrome. Di/di twins have the lowest mortality risk at about nine percent, although that is still significantly higher than that of singletons.^[9]

Monozygotic twins are genetically identical (unless there has been a mutation in development) and they are always the same sex. (On rare occasions, monozygotic twins may express different phenotypes (normally due to an environmental factor or the deactivation of different X chromosomes in monozygotic female twins), and in some extremely rare cases, due to aneuploidy, twins may express different sexual phenotypes, normally due to an XXY Klinefelter's syndrome zygote splitting unevenly ^{[10] [11]}). Monozygotic twins generally look alike, although they do not have the same fingerprints (which are environmental as well as genetic). As they mature, MZ twins often become less alike because of lifestyle choices or external influences. Genetically speaking, the children of MZ twins are half-siblings rather than cousins. If each member of one set of MZ twins marries one member of another set of MZ twins then the resulting children would be genetic full siblings. It is estimated that there are around 10 million monozygotic twins and triplets in the world.

The likelihood of a single fertilisation resulting in MZ twins appears to be a random event, not a hereditary trait, and is uniformly distributed in all populations around the world.^[7] This is in marked contrast to DZ twinning which ranges from about six per thousand births in Japan (almost similar to the rate of MZ twins, which is around 4-5 to 15 and more per thousand in some parts of India^[12] (and up to 24 in the US, which might mainly be due to IVF (in vitro fertilisation). The exact cause for the splitting of a zygote or embryo is unknown.

Monozygotic twins have identical DNA but differing environmental influences throughout their lives affect which genes are switched on or off. This is called epigenetic modification. A study of 80 pairs of human twins ranging in age from three to 74 showed that the youngest twins have relatively few epigenetic differences. The number of epigenetic differences between MZ twins increases with age. Fifty-year-old twins had over three times the epigenetic difference of three-year-old twins. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference.^[13] However, certain characteristics become more alike as twins age, such as IQ and personality.^[14][15] This phenomenon illustrates the influence of genetics in many aspects of human characteristics and behaviour.

A recent theory posits that monozygotic twins are formed after an embryo essentially collapses, splitting the progenitor cells (those that contain the body's fundamental genetic material) in half. That leaves the same genetic material divided in two on opposite sides of the embryo. Eventually, two separate fetuses develop. The research was presented at a meeting of the European Society for Human Reproduction and Embryology in Lyon, France. Utilizing computer software to take photos every two minutes of 33 embryos growing in a laboratory, Dr. Dianna Payne, a visiting research fellow at the Mio Fertility Clinic in Japan, documented for the first time the early days of twin development. Payne also discovered explanation for why in-vitro fertilization techniques are more likely to create twins. Only about three pairs of twins per 1,000 deliveries occur as a result of natural conception, while for IVF deliveries, there are nearly 21 pairs of twins for every 1,000.^[16]

A large number of SNPs spaced throughout the genome have been identified recently in a large project called the HapMap project^[17][18]. The usefulness of the HapMap project and SNP typing and their relevance to society was covered in the 27 October 2005 issue of the leading international science journal Nature (Fig 3).

A large number of genes have been identified that contribute to human disease. These are avaialble from the US National Library of Medicine, which has an impressive range of biological science resources available for free online. Amongst these resources is Online Mendelian Inheritance in Man - OMIM that provides a very, very comprehensive list of all known human gene mutations associated with, and likely contributing to, disease. Each article at OMIM is regularly updated to include the latest scientific research. Additionally, each article provides a detailed history of the research on a given disease gene, with links to the research articles. This resource is highly valuable and is used by the world's top science researchers.

Demographics

A recent study found that vegan mothers are five times less likely to have twins than those who eat animal products.^[19]

From 1980–97, the number of twin births in the United States rose 52%.^[20] This rise can at least partly be attributed to the increasing popularity of fertility drugs like Clomid and procedures like in vitro fertilization, which result in multiple births more frequently than unassisted fertilizations do. It may also be linked to the increase of growth hormones in food.^[19]

Ethnicity

About 1 in 90 human births (1.1%) results from a twin pregnancy.^[21] The rate of fraternal twinning varies greatly among ethnic groups, ranging as high as about 6% for the Yoruba or 10% for Linha Sao Pedro, a tiny Brazilian village.^[22] The widespread use of fertility drugs causing hyperovulation (stimulated release of multiple eggs by the mother) has caused what some call an "epidemic of multiple births". In 2001, for the first time ever in the US, the twinning rate exceeded 3% of all births. Thus, approximately 5.8% of children born in the US in 2001 were twins. Among Hausa of Nigeria and Niger the incidence of multiple births was studied using the maternity records of 5750 Hausa women living in the savannah zone of Nigeria. There were 40 twins and 2 triplets/1000 births. Twenty six per cent of twins were monozygous. The incidence of multiple births, which was about five times higher than that observed in any western population, was significantly lower than that of other ethnic groups, who live in the hot and humid climate of the southern part of country. The incidence of multiple births was related to maternal age but did not bear any association to the climate or prevalence of malaria.^[23] Nevertheless, the rate of identical twins remains at about 1 in 333 across the globe, further suggesting that pregnancies resulting in identical twins occur randomly.

Predisposing factors

The cause of monozygotic twinning is unknown.

Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:

• She is of West African descent (especially Yoruba or Hausa)^[7]
  • She is between the age of 30 and 40 years
  • She is greater than average height and weight
  • She has had several previous pregnancies.
  • She has a family history of dizygotic twinning, especially a mother who is a twin.

  Women undergoing certain fertility treatments may have a greater chance of dizygotic multiple births. This can vary depending on what types of fertility treatments are used. With in vitro fertilisation (IVF), this is primarily due to the insertion of multiple embryos into the uterus. Some other treatments such as the drug Clomid can stimulate a woman to release multiple eggs, allowing the possibility of multiples. Many fertility treatments have no effect on the likelihood of multiple births.

  There is also speculation that the West African predisposition to twinning is due to the large amount of yams in their diet because yams contain phytoestrogens, which may stimulate the ovaries. Most likely this is nothing but a myth, as phytoestrogens would tend to suppress twinning by reducing gonadotropin levels.

  Complications of twin pregnancy

  Vanishing twins

  Main article: Vanishing twin

  Researchers suspect that as many as 1 in 8 pregnancies start out as multiples, but only a single fetus is brought to full term, because the other has died very early in the pregnancy and not been detected or recorded.^[24] Early obstetric ultrasonography exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes. This is known as vanishing twin syndrome.

  

  

  Chang and Eng Bunker, born in Siam (now Thailand) in 1811, were the origin of the term "Siamese twins".

  Conjoined twins

  Main article: Conjoined twin

  Conjoined twins (or "Siamese twins") are monozygotic twins whose bodies are joined together at birth. This occurs where the single zygote of MZ twins fails to separate completely, and the zygote starts to split after day 13 following fertilization. This condition occurs in about 1 in 50,000 human pregnancies. Most conjoined twins are now evaluated for surgery to attempt to separate them into separate functional bodies. The degree of difficulty rises if a vital organ or structure is shared between twins, such as brain, heart or liver.

  Chimerism

  Main article: Chimera (genetics)

  A chimera is an ordinary person or animal except that some of her parts actually came from her twin or from the mother. A chimera may arise either from monozygotic twin fetuses (where it would be impossible to detect), or from dizygotic fetuses, which can be identified by chromosomal comparisons from various parts of the body. The number of cells derived from each fetus can vary from one part of the body to another, and often leads to characteristic mosaicism skin colouration in human chimeras. A chimera may be a hermaphrodite, composed of cells from a male twin and a female twin.

  Parasitic twins

  Main article: Parasitic twin

  Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. One fetus acts as a parasite towards the other.

  Sometimes the parasitic twin becomes an almost indistinguishable part of the other.

  Partial molar twins

  A very rare type of parasitic twinning is where a single viable twin is endangered when the other zygote becomes cancerous, or molar. This means that the molar zygote's cellular division continues unchecked, resulting in a cancerous growth that overtakes the viable fetus. Typically, this results when one twin has either triploidy or complete paternal uniparental disomy, resulting in little or no fetus and a cancerous, overgrown placenta, resembling a bunch of grapes.

  Miscarried twin

  Occasionally, a woman will suffer a miscarriage early in pregnancy, yet the pregnancy will continue; one twin was miscarried but the other was able to be carried to term. This occurrence is similar to the vanishing twin syndrome.

  Low birth weight

  Twins typically suffer from the lower birth weights and greater likelihood of prematurity that is more commonly associated with the higher multiple pregnancies. Throughout their lives twins tend to be smaller than singletons on average.^{[citation needed]}

  Twin-to-twin transfusion syndrome

  Main article: Twin-to-twin transfusion syndrome

  Monozygotic twins who share a placenta can develop twin-to-twin transfusion syndrome. This condition means that blood from one twin is being diverted into the other twin. One twin, the 'donor' twin, is small and anemic, the other, the 'recipient' twin, is large and polycythemic. The lives of both twins are endangered by this condition.

  Human twin studies

  Main article: Twin study

  Twin studies are studies that assess monozygotic twins for medical, genetic, or psychological characteristics to try to isolate genetic influence from environmental influence. Twins that have been separated early in life and raised in separate households are especially sought-after for these studies, which have been invaluable in the exploration of human nature.

  Unusual twinnings

  There are some patterns of twinning that are exceedingly rare: while they have been reported to happen, they are so unusual that most obstetricians or midwives may go their entire careers without encountering a single case.

  Among fraternal twins, in rare cases, the eggs are fertilized at different times with two or more acts of sexual intercourse, either within one menstrual cycle (superfecundation) or, even more rarely, later on in the pregnancy (superfetation). This can lead to the possibility of a woman carrying fraternal twins with different fathers (that is, half-siblings). This phenomenon is known as heteropaternal superfecundation. One 1992 study estimates that the frequency of heteropaternal superfecundation among dizygotic twins whose parents were involved in paternity suits was approximately 2.4%; see the references section, below, for more details.

  Fraternal twins from biracial couples can sometimes be mixed twins - which exhibit differing ethnic and racial features.

  Among monozygotic twins, in extremely rare cases, twins have been born with opposite sexes (one male, one female). The probability of this is so vanishingly small (only 3 documented cases^[25]) that multiples having different sexes is universally accepted as a sound basis for a clinical determination that in utero multiples are not monozygotic. When monozygotic twins are born with different sexes it is because of chromosomal birth defects. In this case, although the twins did come from the same egg, it is incorrect to refer to them as genetically identical, since they have different karyotypes.

  

  

  Twin calves of the Hereford breed.

  Semi-Identical Twins

  Monozygotic twins can develop differently, due to different genes being activated.^[26]. More unusual are "Semi-identical twins". These "half-identical twins" are hypothesized to occur when an unfertilized egg cleaves into two identical attached ova and which are viable for fertilization. Both cloned ova are then fertilized by different sperm and the coalesced eggs undergo further cell duplications developing as a chimeric blastomere. If this blastomere then undergoes a twinning event, two embryos will be formed, each of which have different paternal genes and identical maternal genes.

  This results in a set of twins with identical genes from the mother's side, but different genes from the father's side. Cells in each fetus carry genes from either sperm, resulting in chimeras. This form had been speculated until only recently being recorded in western medicine^{[27] [28] [29]}.

  Animal twins

  Twins are common in many animal species, such as cats, sheep, ferrets and deer. The incidence of twinning among cattle is about 1-4%, and research is underway to improve the odds of twinning, which can be more profitable for the breeder if complications can be sidestepped or managed. A species of armadillo (Dasypus novemcinctus) has identical twins (usually four babies) as its regular reproduction and not as exceptional cases.^{[citation needed]}

  See also

  

  Wikimedia Commons has media related to:

  Twins
  • Evil twin
  • List of twins
  • List of multiple births
  • NICU (Neonatal Intensive Care Unit)
  • Twinless twins

  Notes

  1. ^ Elliott, JP (Dec 2007). "Preterm labor in twins and high-order multiples.". Clinical Perinatology (4): 599-609. PMID 18063108. “Unlike singleton gestation where identification of patients at risk for PTL is often difficult, every multiple gestation is at risk for PTL, so all patients can be managed as being at risk.” 
  2. ^ Oliver, Judith (2006). Twin Resources. Economic and Social Research Council. Retrieved on 2008-02-06.
  3. ^ Rush University Medical Center. Retrieved on 2008-03-06.
  4. ^ The CIA World Fact Book. CIA.gov.
  5. ^ Kris Bigalk. Rare Forms of Twinning. bellaonline.com. Retrieved on 2007-03-22.
  6. ^ R. Derom; J. Orlebeke, A. Eriksson, M. Thiery (1995). "The epidemiology of multiple births in Europe", in Louis G. Keith: Multiple Pregnancy: Epidemiology, Gestation & Perinatal Outcome. Taylor and Francis, 145. ISBN 1850706662. 
  7. ^ ^{a b c} Bortolus, Renata; Fabio Parazzini, Liliane Chatenoud, Guido Benzi, Massimiliano Maria Bianchi, Alberto Marini (1999). "The epidemiology of multiple births". Human Reproduction Update 5 (2): 179-187. European Society of Human Reproduction and Embryology. ISSN 1355-4786. PMID 10336022. 
  8. ^ April Holladay (2001-05-09). What triggers twinning?. WonderQuest. Retrieved on 2007-03-22.
  9. ^ Benirschike K, "Multiple Gestation" in Creasy RK, Resnik R, (eds.) Maternal-Fetal Medicine: Principles and Practice; 5th ed. (2004):55-62. Philadelphia: Saunders.
  10. ^ Edwards, J. H., Dent, K., and Kahn,J. Monozygotic twins of different sex. J. Med. Genet. 1966 3: 117-123.
  11. ^ Machin, G. A. Some causes of genotypic and phenotypic discordance in monozygotic twin pairs. Amer. J. Med. Genet. 1996 61: 216-228
  12. ^ Oleszczuk JJ, Keith DM et al. Projection of Population-based twinning rates through the year 2100. J Reprod Med. 44(11), 1999. 913–921.
  13. ^ Fraga, M.; Ballestar, E. & Paz, M. et al. (2005), "Epigenetic differences arise during the lifetime of monozygotic twins", Proceedings of the National Academy of Sciences 102 (30): 10413—10414
  14. ^ Segal, K. Entwined Lives: Twins and What They Tell Us About Human Behavior, Plume Publishing, 2002
  15. ^ DeFries, J. McGuffin, P., McClearn, G., Plomin, R., Behavioural Genetics, Worth Publishers, 4th Edition, 2000
  16. ^ Associated Press. Study: Twins form after embryo collapses. Retrieved on 2007-07-03.
  17. ^ McVean G, Spencer CC, Chaix R (2005). "Perspectives on human genetic variation from the hapmap project". PLoS Genet 1 (4): e54. PMID 16254603. This review is free of charge
  18. ^ Skelding K.A., Gerhard GS, Simari RD, Holmes DR Jr (2007). "The effect of HapMap on cardiovascular research and clinical practice". Nat Clin Pract Cardiovasc Med 4 (3): 136-142. PMID 17330125. 
  19. ^ ^{a b} Gary Steinman, M.D., Ph.D. Mechanisms of Twinning: VII. Effect of Diet and Heredity on the Human Twinning Rate. Journal of Reproductive Medicine May 2006.
  20. ^ National Vital Statistics Reports Vol. 47, No. 24: "Trends in Twin and Triplet Births: 1980-1997" (PDF)
  21. ^ Asch, Ricardo (1995). Progress in Reproductive Medicine. Informa Health Care. ISBN 1850705747. 
  22. ^ U. Matte et al. Study on possible increase in twinning rate at a small village in south Brazil. Acta Genet Med Gemellol (Roma). 45(4), 1996. 431–437.
  23. ^ "MULTIPLE BIRTHS IN HAUSA WOMEN". "An International Journal of Obstetrics and Gynaecology".
  24. ^ L. Gedda (1995). "The role of research in twin medicine.", in Louis G. Keith: Multiple Pregnancy: Epidemiology, Gestation & Perinatal Outcome. Taylor and Francis, 4. ISBN 1850706662. 
  25. ^ [1]
  26. ^ Non-identical Monozygotic Twins
  27. ^ Vivienne L. Souter, Melissa A. Parisi, Dale R. Nyholt, Raj P. Kapur, Anjali K. Henders, Kent E. Opheim, Daniel F. Gunther, Michael E. Mitchell, Ian A. Glass and Grant W. Montgomery, A case of true hermaphroditism reveals an unusual mechanism of twinning, Human Genetics journal (Vol 121, No 2, pp179-185), April 2007; ISSN 0340-6717 (Print), 1432-1203 (Online)
  28. ^ Rare Semi-Identical Twins Discovered
  29. ^ 'Semi-identical' twins discovered from Nature

  
  

  References

  1. Nieuwint A, Van Zalen-Sprock R, Hummel P, Pals G, Van Vugt J, Van Der Harten H, Heins Y, Madan K. (1999). "'Identical' twins with discordant karyotypes". Prenatal Diagnosis 19 (1): 72-6. PMID 10073913. 
  2. Wenk RE, Houtz T, Brooks M, Chiafari FA (1992). "How frequent is heteropaternal superfecundation?". Acta geneticae medicae et gemellologiae 41 (1): 43-7. PMID 1488855. 
  3. Girela E, Lorente JA, Alvarez JC, Rodrigo MD, Lorent M, Villaneuva E (1997). "Indisputable double paternity in dizygous twins". Fertility and Sterility 67 (6): 1159-61. PMID 9176461. 

  
  

  Further reading

  • Schein, E., Bernstein, P. (Random House, 2007). "Identical Strangers: A Memoir of Twins Separated and Reunited."
  • Helle, S., Lummaa, V. & Jokela, J. (2004). Selection for increased brood size in pre-industrial humans. Evolution 52: 430-436. Full textca:Bessó

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